Semiconductor devices comprising a heterojunction

ABSTRACT

LAYER OF THE HALIDE OF THE II MATERIAL IS SUBSEQUENTLY REMOVED WITH A SOLVENT THEREOF.   A METHOD OF FORMING A HETEROJUNCTION IN A SEMICONDUCTOR DEVICE WHEREIN A SUBSTRATE OF A POLYCRYSATLLINE LAYER OF A II-VI MATERIAL COATED WITH A THIN LAYER OF A HALIDE OF COPPER, SILVER AND/OR GOLD IS HEATED TO EFFECT A SOLID-STATE REACTION WHEREIN A HALIDE LAYER OF THE II-VI MATERIAL IS FORMED IN THE THIN LAYER AND AT LEAST ONE OF THE METALS COPPER, SILVER, AND GOLD PENETRATES INTO THE SUBSTRATE TO FORM A COMPOUND WITH THE VI MATERIAL WHICH PROVIDES A HETEROJUNCTIN WITH THE SUBSTRATE. THE THIN

July 25, T. 5. TE VELDE HAL SEMICONDUCTOR DEVICES COMPRISING AHETEROJUNCTION Filed Jan. 27. 1970 Y 1 111/ 17/114 J10 f \\\m H92 fig.3

INVENTORS TIES 8. TE VELDE and BY SYBRANDUS VAN HEUSDEB United StatesPatent Oflice 3,679,496 Patented July 25, 1972 3,679,496 SEMICONDUCTORDEVICES COMPRISING A HETEROJUNCTION Ties Siebolt Te Velde and Sybrandusvan Heusden, Emmasingel, Eindhoven, Netherlands, assignors to US.Philips Corporation, New York, NY.

Filed Jan. 27, 1970, Ser. No. 6,066 Claims priority, applicationNetherlands, Feb. 1, 1969, 6901662 Int. Cl. H01] 7/44 US. Cl. 148188 11Claims ABSTRACT OF THE DISCLOSURE A method of forming a heterojunctionin a semiconductor device wherein a substrate of a polycrystalline layerof a IIVI material coated with a thin layer of a halide of copper,silver and/ or gold is heated to effect a solid-state reaction wherein ahalide layer of the lI-VI material is formed in the thin layer and atleast one of the metals copper, silver, and gold penetrates into thesubstrate to form a compound with the VI material which provides aheterojunction with the substrate. The thin layer of the halide of theII material is subsequently removed with a solvent thereof.

The invention relates to a method of manufacturing a semiconductordevice in which a body consisting at least on one side at least partlyof a first compound of the type II-VI (a so called II-VI compound) isprovided with a second compound of at least one of the metals Cu, Ag andAu and at least one of the metalloids of said II-VI compound, forming ahetero-junction with the II-VI compound, to a semiconductor device thusmanufactured and to a solar battery comprising at least one of suchsemiconductor devices. Whereas with a homo-junction in a semiconductorthe material on either side of the junction is of the same chemicalsubstance having the same crystal structure (though differently doped),the material on either side of a hetero-junction is essentiallydifferent either in chemical nature or in crystal structure or in both.

Such hetero-junctions, provided their structure is Well defined, mayhave interesting electrical properties so that they may be employed foran effective injection or extraction of charge carriers particularly insemiconductors in which satisfactory homo-junctions can be provided onlywith difiiculty. Because, in addition, the optical properties of thesemiconductor materials on either side of interface at thehetero-junction may be highly different, the hetero-junction oftenprovides more possibilities than a homo-junction in devices in which aneffective excitation or emission of light has to be obtained. Examplesthereof are the uses of hetero-junctions in solar batteries orelectro-luminescent devices based on II-VI compounds.

However, hitherto the efliciency of said devices was not optimal becausethe existing methods of applying the hetero-junctions did not result inan adequately defined construction. It is known, for example, to treat aCdS body with a solution of 0150 the exchange of Cdand Cu-ions providinga Cu S layer on the CdS, which forms a hetero-junction therewith. Inother methods hitherto employed metallic Cu, Ag or Au was appliedgalvanically or from the vapour phase to a II-VI compound, after which,by heating in a given atmosphere, by means of a chemical reaction also ahetero-junction may be formed between the resultant chalcogenide of theapplied metal and the body of the (II-VI) compound. These known methodshave the following disadvantages: the layer of the material forming thehetero-junction may be doped only with difficulty and after the requiredthermal treatments its structure and thickness are not uniform, whilstat the area of the junction a great, insufficiently controlled densityof surface levels has in general an adverse effect on the operation ofthe junction.

According to the invention these disadvantages are avoided by applying,in the method of the kind set forth in the preamble, a layer of a thirdcompound, being a halogen compound of at least one of said metals Cu, Agand Au, to the semiconductor body, after which by heating, a solid-statereaction between the halide (the third compound) and the I IVI compoundproduces the second compound forming the hetero-junction, after whichthe resultant fourth compound of the metal of the II-VI compound and atleast one of the halogens is removed by dissolving it.

It is preferred to apply monovalent halogen compounds of the metalsconcerned, particularly for obtaining a satisfactory photo-voltaiceffect. In order to obtain a flat hetero-junction at a uniform depthbeneath the surface, particularly at a very small depth, the layer ofthe halogen compound is preferably applied by evaporation.

The invention will now be described more fully by way of example andwith reference to the accompanying drawing.

FIG. 1 shows, by way of example, vertical sectional views of threedifferent starting forms of semiconductor members for the manufacture ofsemiconductor devices with hetero-junctions. All these bodies compriseII-VI compounds, for example, chalcogenides of the bivalent metals Zn,Cd or Hg.

FIG. 2. shows a vertical cross-section of a semiconductor deviceprepared by the method according to the inventron.

FIG. 3 is a graph showing current-voltage characteristics of the deviceshown in FIG. 2.

According to FIG. 1a the starting member is a plateshaped single crystal1 of a II-VI compound, for ex ample, CdS.

According to FIG. 1b the starting member is a so-called monograin layer,in which crystal grains 3 of a II-VI compound are embedded, in themanner shown, in a film 5 of a synthetic resin for example polyurethane,surface parts of the grains 3 being free from the resin at both sides ofthe monograin layer.

According to FIG. 10 the starting member is a polycrystalline layer 6 ofII-VI material applied to a substrate 8 of, for example, glass by vapourdeposition.

In a first step of the method in accordance with the in vention such abody is provided or coated with a thin layer (2, 4 and 9 of FIGS. 1a, 1band 1c respectively) of a halide of Cu, Ag and/or Au, for example, CuCl,by evaporation in vacuo, the II-VI body being substantially at roomtemperature. The temperature of the vaporizing source is adjusted sothat a constant vapour flow is obtained. In the case of CuCl thevaporization vessel was heated at 600 C. for a few minutes. Such adeposited CuCl layer may have a preferred thickness between 0.05

and 2p, for example, 0.2;. The layer material substantially does notreact with the II-VI substrate (which may consist of CdS) during thevapour deposition process. The layer thus provided has a well definedgeometry, for example, it has a uniform thickness.

The member provided with the CuCl-layer is then subjected to a heatingprocess, preferably between 100 C. and 400 0., preferably for 1 to 30minutes, for example, for 3 minutes, the temperature being maintained,for example, at 150 C. A reducing atmosphere, for example, of hydrogenmay be used, but a neutral atmosphere, for example, of nitrogen or arare gas may also be employed, whilst a small content of oxygen orhydrogen is permissible. It is found that under these conditions asolid-state reaction occurs, in which copper penetrates to a welldefined depth into the CdS and forms cuprous sulphide, whilst at thesame time the cadmium migrates out through the same layer thickness overwhich said reaction takes place, forming a layer of cadmium chloridewhich occupies about the same space as the initially vapour-depositedcopper chloride. This CdCl, layer is subsequently removed by means of asuitable solvent, for example, an alcohol or water. The surface thusexposed is found to have maintained the original structure of the II-VIsurface; this is a result of the fact that in said solid-state reactionthe metalloid ions forming a close-packed structure in the crystalstructure of the II-VI body have not appreciably changed in place.

Such an assembly of cuprous sulphide and CdS already operates as ahetero-junction. The plane formed by the junction, i.e. the interfacebetween the two different substances, may be defined experimentally bydissolving the cuprous sulphide selectively in a KCN solution. Thisinterface appears to have a very well defined structure with atmost afew unevennesses. It is located at a depth beneath the initial II-VIsurface which is slightly smaller (5 to than the layer thickness of thevapour-deposited halide and the plane of the junction extendssubstantially accurately parallel to the initial lI-VI surface. Theseparticularities are already proof of the special advantages of themethod according to the invention as compared with the known methodsmentioned before, by means of which it is practically impossible toobtain a hetero-junction of such a well defined structure. Therefore, inorder to avoid subsequent short-circuits in the methods hitherto used aconsiderably thicker II-VI body had to be used at the start than isrequired in the novel method.

The electrical properties of the above described heterojunction may beconsiderably influenced by a thermal after-treatment. The yield of thephoto-voltaic effect in such a junction may be further enhanced bysubjecting the assembly, subsequent to dissolution of the halide formed,to a tempering treatment at a temperature lying between I150 C. and 3000., preferably for at least one minute, for example, for a few minutesat 180 C. It is preferred to use a neutral atmosphere, for example,consisting in this case, of nitrogen, to which traces of O, and/or H Oare added (for example in a concentration of about 1%). For themanufacture of junctions having optimal rectification properties withoutillumination a slightly difierent thermal after-treatment may be used.Heating may be carried out, for example, for one minute at 100 C. in(NI-[0 S vapour so that the composition of the cuprous sulphide layer isshifted towards a higher value of the atomic ratio of sulphur copper. Adifferent means for promoting this shift from Cu,S towards CuS consistsin forming a layer of cupric chloride or cuprousand cupricchloride, forexample, by treating the vapour-deposited cuprous chloride layer withchlorine gas, as a result of which the copper compound taking part inthe subsequent solid-state reaction is already less rich in copper.

FIG. 2 shows a semiconductor device, more specifically a photo-cell,based on a single crystal of CdS 10 manufactured by the method accordingto the invention. The cuprous sulphide layer 11 is locally providedwith'a con- 4 tact 12 by means of a conductive silver paste, whereas therear side of the CdS crystal is provided with an indium contact 13 byvapour deposition. The c-axis of the hexagonal crystal plate is at rightangles to the plate surface and hence also at right angles to the planeof the hetero-junction. The current-voltage characteristics of thisphoto-cell are shown by the curves in the graph of FIG. 3 (i is thecurrent density in Ina/cm V is the voltage between the contacts 12 and13 of FIG. 2). The curve 21 relates to the unexposed state. The curve 22relates to an exposure to radiation having a density of 100 mwJcm. froma light source having a radiation temperature of 3000 C., whichsubstantially corresponds to direct, solar exposure at right angles. Thecopper sulphide layer had been applied to the cadmium side of the CdScrystal plate. The curves 23 and 24 relate in a similar manner to a cellof the same kind,

the copper chloride being applied, however, to the sulphur side of theCdS crystal plate, the curve 23 relating to the unexposed state and thecurve 24 to a similar illumination as in the case of curve 22. The polarnature of the hexagonal crystal structure of CdS comes to light in adifference between the open voltages and short-circuit cur- V Inparticular, when monograin layers of lI-VI grains embedded in an organicbinder (see FIG. lb) are used, the method in accordance with theinvention provides photo-voltaic cells of high quality.

What is claimed is:

1. A method of manufacturing a semiconductor device which comprises:providing a substrate of a poly-crystalline layer of a II-VI material,coating said substrate with a thin layer of a halide of at least onemetal selected from the group consisting of copper, silver and gold,heating said thus coated substrate to produce a solid state reactionbetween said coating and said substrate wherein a layer of a halide ofsaid ILVI material is formed on the surface of said thin layer, and saidat least one metal is caused to penetrate below the surface of saidsubstrate to form a compound with the VI portion of said lI-VI materialbetween said substrate and said thin layer, and removing said layer ofsaid halide of the H portion of said Il-VI material by means of asolvent thereof, whereby a hetero-junction is formed between saidsubstrate and said newly formed compound of said VI material.

2. A method as claimed in claim 1 wherein said thin layer of said halideis applied by vapour deposition.

3. A method as claimed in claim 2, wherein the thickness of thedeposited halide layer lies between 0.05 and 2/p..

4. A method as claimed in claim 1, wherein said solidstate reactionoccurs at a temperature lying between C. and 400 C.

-5. A method as claimed in claim 4, wherein the duration of heating forsaid solid-state reaction lies between 1 minute and 30 minutes.

6. A method as claimed in claim 1 wherein said solvent for dissolvingand removing said halide after the solidstate reaction, includes waterand an alcohol. I

7. A method as claimed in claim 1, wherein the semiconductor bodyprovided with the hetero-junction is subjected to an annealing processat a temperature lying between C. and 300 C. subsequent to said removingstep.

8. A method as claimed in claim 7, wherein the annealmg process iscarried out in a substantially inert atmosphere containing traces of agas selected from the References Cited group consisting of oxygen andhydrogen. UNITED STATES PATENTS 9. A method as claimed claill'l 7,wherein the dura' 3, 4 3 tion of the annealing process is at least oneminute. 5 2,820,841 1/ 1958 Carlson et al. l48186 .10. A method asclaimed in claim 1, wherein the \iI-VI 2,844,640 7/ 1958 y s 148-15 UXmaterial of said substrate consists mainly of CdS and said g i gzz z: 2;

halide applied thereto consists mainly of CuCl. 4

11. A method as claimed in claim- 1, in which said 10 DEWAYNE RUTLEDGE,Primary Examine! substrate comprises a monograin layer composed of a J.M. DAVIS, Assistant Examiner one gram .thlOk layer of grains of sa1dII-VI mater-1 1 Us. Cl. XR. embedded m a film of a synthetic resin. 5

